As a background technique of the present technical field, Patent Document 1 is cited. In the publication, in a Multi Scale Retinex process, a composite blur image is created by selecting any one of a plurality of blur images which are generated from a plurality of peripheral functions different scales and which are different in a degree of the blur for each pixel in accordance with a pixel value level of an original image to be a processing target. It is described that the composite blur image is subjected to low pass filter, so that the Retinex process is performed while occurrence of unnatural discontinuation in a boundary is prevented (see Abstract).